In his statement, Newell said what they had found out so far was that sometime after September this year, someone other than him had been accessing his email account.

Soon after this, he said his computer had started acting up but he was unable to find any virus, worm or trojan to explain this behaviour. He then reformatted the hard drive and reinstalled Windows.

"For the next week, there appears to have been suspicious activity on my webmail account," Newell wrote. And around September 19, someone had made a copy of the Half Life-2 source tree.

"At some point, keystroke recorders got installed on several machines at Valve. Our speculation is that these were done via a buffer overflow in Outlook's preview pane. This recorder is apparently a customised version of RemoteAnywhere created to infect Valve - at least it hasn't been seen anywhere else, and isn't detected by normal virus scanning tools," he said.

Newell said he was seeking assistance to track down the perpetrator(s). "If you have information about the denial of service attacks or the infiltration of our network, please send the details. There are some pretty obvious places to start with the posts and records in IRC (internet relay chat), so if you can point us in the right direction, that would be great," he wrote.